This invention deals with multiple simultaneous tone decoding and more particularly with multiple simultaneous tone decoding at a remote location such as a radio transmitter site. The coded tones are sent from a dispatch point to a transmitter decoder which serves to control the transmitter site operation.
This invention is related to U.S. Pat. No. 3,577,080 to Cannalte and is herein incorporated by reference. In the Cannalte patent a short burst of a high amplitude "guard" tone is applied over a single audio channel from a dispatch point to a remote transmitter decoder for the purpose of activating a control function at the transmitter site. After the transmitter decoder has received a high level guard tone audio signal, the dispatch point then transmits different tones (function tones) over the audio channel to actuate different control functions within the transmitter.
The invention is an improved implementation of the transmitter decoder using the control signalling system of the Cannalte patent. According to the signalling system when a dispatch point wants to send a command to a remote transmitter station, it sends a two tone sequence via a wire line path. As mentioned, the first tone is referred to as a high level guard tone. It is a fixed frequency and serves to prepare the transmitter to receive a second tone. The second tone is commonly referred to as a function tone. Unlike the guard tone, the function tone can be one of many different frequencies. Each function tone frequency signifies a unique command when received by the transmitter. Since the transmitter decoder does not know which function tone will be sent after it receives a high level guard tone, the prior art transmitter decoder uses a separate decoder circuit for each of the possible defined function tones. The need to replicate a tone decoder for every tone has many disadvantages, some of them being high cost, large size, high parts count and components which are highly sensitive to changes in the ambient environment. These components need to be manually tuned and have been shown to drift with time, vibration and temperature. Also, each of the tone decoders operate independently and it is therefore possible for more than one of the multiple tone decoders to simultaneously indicate a detection of an associated tone thus creating undefined fault conditions.
It is the object of this invention to provide a single tone decoder to decode all of the possible function tones.
It is a further object of the invention to provide a tone decoder which will choose only the strongest tone present.
It is also an object of this invention to provide a decoder which utilizes both the average period of a sampled portion of the received tone and the variance of each period within the sample as an indication of valid detection of a function tone.